1. Field of the Invention
The invention refers to the manufacture of metallic patterns or structures upon a flexible substrates using the method of laser ablation and the use of the resulting metallic patterned substrates for RFID straps and RFID antennas.
2. Description of Prior Art
The name RFID stands for Radio Frequency Identification and is used in specially developed systems for automatic identification of items, such as goods or persons or both, using radio transmission. An RFID system comprises a transponder and a reading device. The transponder comprises an integrated circuit microchip (chip) and an antenna. In its simplest form, the transponder is made by forming the antenna on a substrate and electrically connecting the chip to the antenna. The transponder is also known as an RFID inlay.
An RFID tag or RFID label, is a label or a tag having an embedded RFID inlay. Common commercial systems often comprise labelling (e.g., paper label materials), an inlay, a liner (carrier substrate for the inlay) and other application-specific packaging components such as printed surfaces for the labelling. See FIG. 1.
The continued and future successful commercialization of RFID systems depends upon low cost mass production processes. The prior art methods for manufacturing RFID antennas and RFID straps have reached their limits in regards to high volume production.
The case of RFID antennas provides an example of the aforementioned limitation. Antennas are usually manufactured from such materials as copper (Cu), aluminium (Al), silver (Ag), carbon in the form of graphite (C), or a conducting polymer, such as polythiophene. Accordingly, various standard technologies have been developed to process these materials into RFID antennas, for example, photo-lithographic or etching techniques, printing techniques (screen printing, engrave printing, flexo-printing), inkjet printing, galvanic techniques, sputter technologies.
There is an increasing demand for antennas to accommodate higher frequencies (for example 2.45 GHz as opposed to frequencies in the MHz range), to be more selective, to have longer transmission ranges and to have smaller dimensions. For example, in a passive RFID system, increasing the number of windings in a loop antenna of a small RFID inlay allows the tag to capture more energy to power the chip and transmit its information to an RFID reader. These antennas must be more precise and thus have less tolerance for variations in winding or loop widths. For examples of such antennas, see www.fractalantenna.com.
The above mentioned conventional technologies, however, have reached their tolerance limits and cannot manufacture such exacting antennas in a continuous process, such as in a reel-to-reel device, and therefore, these conventional technologies are not suitable for low-cost production, i.e., mass production, of these precise antennas.
Likewise current practice has reached a limit in regards to the speed in the manufacture of inlays. As discussed above, the inlay, i.e., transponder, comprises an antenna and a microchip which are electrically connected. A number of different techniques have been developed to connect the chip to the antenna, such as the flip-chip process. In this process, a robotic arm picks-up and places the chip onto the antenna. The electrical connection is achieved using isotropic or anisotropic adhesives. This process is normally performed under clean room conditions to limit contamination of the electrical connections. The placement of the chips upon the antennas must be very precise and accurate, because the dimensions of the chip's bumps, i.e., the electrical connection points of the chip, are smaller than 50 μm. Accordingly, because of the required precision and clean room conditions required, the flip-chip process represents a critical and limiting path to the high volume and high speed production of transponders.
Because of the limitations of the flip-chip process, the use of an “interposer”, also called a “strap,” was developed to speed up the production of inlays. A strap is a carrier for the chip. In this technique, the chip is conductively connected to a flexible substrate coated with an electrically conductive material such as a metal, i.e., the strap, and in turn, the chip-strap assembly is conductively connected to the RFID antenna. The flexible substrate may comprise an insulating polymer such as polyester, polycarbonate, polyimide, liquid crystal polymer (LCP), among others. One side of the substrate is selectively covered with metal structures. The microchip is thus electrically connected to the metal structures of the strap using conventional techniques of direct chip attachment. Such techniques are already known in the art and not discussed here.
The strap-chip assembly is then bonded to the antenna using conventional assembly techniques, thereby forming the transponder. Unlike the “direct chip attachment” to the antenna method, the strap methods require a much lower level of precision during assembly because the electrical contacts between the strap and antenna have dimensions which are in the mm range (compared with the smaller dimensions of chip's bumps). As a result it is possible to realise an assembly process in which the strap is connected with the antenna using mass production techniques, such as the use of a reel to reel device.
The bonding of the electrical contacts (pads) of the strap to the microchip, however, requires the utmost dimensional precision. Previously the conductive structures of the straps have been produced using techniques such as those discussed above, namely, screen print, ink jet, photo-lithographic techniques, and etching. These techniques have the disadvantage of offering only limited precision in terms of structural resolution. In a reel-to-reel screen print process using a special rotation screen print technique and speeds of approximately 100 meter per minute, the bonding geometries have a precision of approximately greater than 100 μm. Although photolithographic processes are able to work in dimensions of less than 100 μm, they are too slow for the reel-to-reel process because of the step and repeat procedures demanded during exposure, which increases costs. A further aspect requires illustration at this juncture. In order to achieve high assembly rates for microchips on straps when implementing a reel-to-reel technique, a key factor is the precision of the structures placed on the straps layout (arrangement of the conductive structures on the substrate) and the precision of the repeated replication of the individual layouts (i.e. distance of one interposer to the next interposer). Only by achieving high precision is it possible to reliably realise maximum accuracies and throughputs in the assembly process. Therefore, optimal image identification and replication and minimal readjustment at point of placement are crucial and decisive factors in the overall production rate of the assembly process.